1. Field of Invention
The invention relates to cryptographic systems.
2. Description of Prior Art
Cryptographic systems are widely used to ensure the privacy and authenticity of messages communicated over insecure channels. A privacy system prevents the extraction of information by unauthorized parties from messages transmitted over an insecure channel, thus assuring the sender of a message that it is being read only by the intended receiver. An authentication system prevents the unauthorized injection of messages into an insecure channel, assuring the receiver of the message of the legitimacy of its sender.
Currently, most message authentication consists of appending an authenticator pattern, known only to the transmitter and intended receiver, to each message and encrypting the combination. This protects against an eavesdropper being able to forge new, properly authenticated messages unless he has also stolen the cipher key being used. However, there is little protection against the threat of dispute; that is, the transmitter may transmit a properly authenticated message, later deny this action, and falsely blame the receiver for taking unauthorized action. Or, conversely, the receiver may take unauthorized action, forge a message to itself, and falsely blame the transmitter for these actions. The threat of dispute arises out of the absence of a suitable receipt mechanism that could prove a particular message was sent to a receiver by a particular transmitter.
One of the principal difficulties with existing cryptographic systems is the need for the sender and receiver to exchange a cipher key over a secure channel to which the unauthorized party does not have access. The exchange of a cipher key frequently is done by sending the key in advance over a secure channel such as private courier or registered mail; such secure channels are usually slow and expensive.
Diffie, et al, in "Multiuser Cryptographic Techniques," AFIPS-Conference Proceedings, Vol. 45, pp. 109-112, June 8, 1976, propose the concept of a public key cryptosystem that would eliminate the need for a secure channel by making the sender's keying information public. It is also proposed how such a public key cryptosystem could allow an authentication system which generates an unforgeable message dependent digital signature. Diffie presents the idea of using a pair of keys E and D, for enciphering and deciphering a message, such that E is public information while D is kept secret by the intended receiver. Further, although D is determined by E, it is infeasible to compute D from E. Diffie suggests the plausibility of designing such a public key cryptosystem that would allow a user to encipher a message and send it to the intended receiver, but only the intended receiver could decipher it. While suggesting the plausibility of designing such systems, Diffie presents neither proof that public key cryptosystems exist, nor a demonstration system.
Diffie suggests three plausibility arguments for the existence of a public key cryptosystem: a matrix approach, a machine language approach and a logic mapping approach. While the matrix approach can be designed with matrices that require a demonstrably infeasible cryptanalytic time (i.e., computing D from E) using known methods, the matrix approach exhibits a lack of practical utility because of the enormous dimensions of the required matrices. The machine language approach and logic mapping approach are also suggested, but there is no way shown to design them in such a manner that they would require demonstrably infeasible cryptanalytic time.
Diffie also introduces a procedure using the proposed public key cryptosystems, that could allow the receiver to easily verify the authenticity of a message, but which prevents him from generating apparently authenticated messages. Diffie describes a protocol to be followed to obtain authentication with the proposed public key cryptosystem. However, the authentication procedure relies on the existence of a public key cryptosystem which Diffie did not provide.